Translucent ceramics such as alumina are used for a light emitting bulb of a metal halide lamp. As shown in FIG. 10, a light emitting portion 31a and tubular portions 31b of the light emitting bulb are molded in one piece to increase heat efficiency and improve lamp characteristics.
A metal halide lamp that includes an integrally molded light emitting bulb is shown in FIG. 11 and a sectional view of this light emitting bulb is shown in FIG. 12.
The light emitting bulb 31 is placed in an outer bulb 32 and lead wires 33a and 33b are attached to the tubular portions of the light emitting bulb 31. The lead wires 33a and 33b are connected to electrodes 17a and 17b in the bulb 31. One end of the outer bulb 32 is closed by a base 34. A stem 36 holds stem leads 35a and 35b, and one of the stem leads 35a is connected to the lead wire 33a and the other stem lead 35b is connected to the lead wire 33b via a power supply wire 37.
Mercury and an iodide of cerium, sodium, thallium, indium, or scandium are sealed in the light emitting bulb 31. When turning on the lamp, the mercury is heated and vaporized by an electric discharge between the electrodes 17a and 17b. Then, the iodide is liquefied and a part of the iodide is vaporized. However, another part of the iodide is attached to the inner surface of the light emitting bulb 31 in a liquid state. When a longitudinal direction of the bulb 31 is arranged along a vertical direction as in FIG. 12 and the lamp is turned on, the liquid iodide is dropped into a gap between the inner surface of the lower tubular portion and the electrode 17a. The liquid iodide 38 in the tubular portion reduces a vapor pressure of iodide in the bulb and causes a change in lamp characteristics, especially in color temperature.